Diesel engines have been used in large vehicles, locomotives and ships as more efficient alternatives to the gasoline engines. They have also been used in small passenger vehicle applications, but have not been as popular as in other applications until recently. The two main factors that kept them from becoming the major contender in the small passenger vehicle applications were the low power outputs and the noise levels.
A combination of improved mechanical technologies such as multiple injection, higher injection pressure, and advanced electronic control has mostly mitigated the problems associated with the noise level and changed the public notion of the Diesel engine technology in the latest generation of common-rail designs. The power output of the Diesel engines has also been improved substantially through the use of variable geometry turbines combined with the advanced fuel injection technology. However, recent trend in automotive industry suggests that the dual-stage boosting is also essential to further improve the power output of the Diesel engine to the level comparable to that of the other engine technologies. The advantage of the dual-stage boosting system over the single-stage system is the increase in the rated output while simultaneously improving the steady-state torque at low engine speeds and the transient response of the Diesel engine by rapidly building up boost pressure.
In this study, several different types of dual-stage boosting systems are evaluated with a physics-based zero-dimensional Diesel engine system simulation in terms of their steady-state and transient performance characteristics and fuel economy improvements. The dual-stage boosting systems evaluated in the study include a boosting system with two fixed geometry turbochargers at both the high pressure and low pressure stages, and several hybrid boosting systems in which the high pressure turbocharger is replaced with a screw type supercharger, an electrical compressor, and a variable geometry turbocharger. A dual-stage boosting system with early intake valve closing (EIVC) strategy is also evaluated. Each alternative system exhibits unique tradeoffs and improvements in terms of performance and fuel economy compared to the dual-stage boosting system with fixed geometry turbochargers at both high and low stages.
